New developments in drug discovery and development are creating new demands on analytical techniques. For example, combinatorial chemistry is often employed to discover new lead compounds, or to create variations of a lead compound. Combinatorial chemistry techniques can generate thousands or millions of compounds (combinatorial libraries) in a relatively short time (on the order of days to weeks). Testing such a large number of compounds for biological activity in a timely and efficient manner requires high-throughput screening methods which allow rapid evaluation of the characteristics of each candidate compound.
The compounds in combinatorial libraries are often tested simultaneously against a molecular target. For example, an enzyme assay employing a calorimetric measurement may be run in a 96-well plate. An aliquot of enzyme in each well is combined with tens or hundreds of compounds. An effective enzyme inhibitor will prevent development of color due to the normal enzyme reaction, allowing for rapid spectroscopic (or visual) evaluation of assay results. If ten compounds are present in each well, 960 compounds can be screened in the entire plate, and one hundred thousand compounds can be screened in 105 plates, allowing for rapid and automated testing of the compounds.
Often, however, determination of which compounds are present in certain portions of a combinatorial library is difficult, due to the manner of synthesis of the library. For example, the "split-and-pool" method of random peptide synthesis in U.S. Pat. No. 5,182,366, describes a way of creating a peptide library where each resin bead carries a unique peptide sequence. Placing ten beads in each well of a 96-well plate, followed by cleavage of the peptides from the beads and removal of the cleavage solution, would result in ten (or fewer) peptides in each well of the plate. Enzyme assays could then be carried out in the plate wells, allowing 100,000 peptides to be screened in 105 plates. However, the identity of the peptides would not be known, requiring analysis of the contents of each well.
The peptides could be analyzed by removing a portion of solution from each well and injecting the contents into a separation device such as liquid chromatography or capillary electrophoresis instrument coupled to a mass spectrometer. Assuming that such a method would take approximately 5 minutes per analysis, it would require over a month to analyze the contents of 105 96-well plates, assuming the method was fully automated and operating 24 hours a day.
This example illustrates the critical need for a method for rapid analysis of large numbers of compounds or complex mixtures of compounds, particularly in the context of high-throughput screening. Techniques for generating large numbers of compounds, for example through combinatorial chemistry, have been established. High-throughput screening methods are under development for a wide variety of targets, and some types of screens, such as the calorimetric enzyme assay described above and ELISA (enzyme linked immunosorbent assay) technology, are well-established. As indicated in the example above, a bottleneck often occurs at the stage where multiple mixtures of compounds, or even multiple individual compounds, must be characterized.
This need is further underscored when current developments in molecular biotechnology are considered. Enormous amounts of genetic sequence data are being generated through new DNA sequencing methods. This wealth of new information is generating new insights into the mechanism of disease processes. In particular, the burgeoning field of genomics has allowed rapid identification of new targets for drug development efforts. Determination of genetic variations between individuals has opened up the possibility of targeting drugs to individuals based on the individual's particular genetic profile. Testing for cytotoxicity, specificity, and other pharmaceutical characteristics could be carried out in high-throughput assays instead of expensive animal testing and clinical trials. Detailed characterization of a potential drug or lead compound early in the drug development process thus has the potential for significant savings both in time and expense.
Development of viable screening methods for these new targets will often depend on the availability of rapid separation and analysis techniques for analyzing the results of assays. For example, an assay for potential toxic metabolites of a candidate drug would need to identify both the candidate drug and the metabolites of that candidate. An assay for specificity would need to identify compounds which bind differentially to two molecular targets such as a viral protease and a mammalian protease.
It would therefore be advantageous to provide a method for efficient proteomic screening in order to obtain the pharmacokinetic profile of a drug early in the evaluation process. An understanding of how a new compound is absorbed in the body and how it is metabolized can enable prediction of the likelihood for an increased therapeutic effect or lack thereof.
Given the enormous number of new compounds that are being generated daily, an improved system for identifying molecules of potential therapeutic value for drug discovery is also critically needed.
It also would be desirable to provide rapid sequential analysis and identification of compounds which interact with a gene or gene product that plays a role in a disease of interest. Rapid sequential analysis can overcome the bottleneck of inefficient and time-consuming serial (one-by-one) analysis of compounds.
Accordingly, there is a critical need for high-throughput screening and identification of compound-target reactions in order to identify potential drug candidates.
Microchip-based separation devices have been developed for rapid analysis of large numbers of samples. Compared to other conventional separation devices, these microchip-based separation devices have higher sample throughput, reduced sample and reagent consumption and reduced chemical waste. The liquid flow rates for microchip-based separation devices range from approximately 1-300 nanoliters (nL) per minute for most applications.
Examples of microchip-based separation devices include those for capillary electrophoresis (CE), capillary electrochromatography (CEC) and high-performance liquid chromatography (HPLC). See Harrison et al, Science 1993, 261, 859-897; Jacobson et al. Anal. Chem. 1994, 66, 1114-1118; and Jacobson et al. Anal. Chem. 1994, 66, 2369-2373. Such separation devices are capable of fast analyses and provide improved precision and reliability compared to other conventional analytical instruments.
Liquid chromatography (LC) is a well-established analytical method for separating components of a fluid for subsequent analysis and/or identification. Traditionally, liquid chromatography utilizes a separation column, such as a cylindrical tube, filled with tightly packed beads, gel or other appropriate particulate material to provide a large surface area. The large surface area facilitates fluid interactions with the particulate material, and the tightly packed, random spacing of the particulate material forces the liquid to travel over a much longer effective path than the length of the column. In particular, the components of the fluid interact with the stationary phase (the particles in the liquid chromatography column) as well as the mobile phase (the liquid eluent flowing through the liquid chromatography column) based on the partition coefficients for each of the components. The partition coefficient is a defined as the ratio of the time an analyte spends interacting with the stationary phase to the time spent interacting with the mobile phase. The longer an analyte interacts with the stationary phase, the higher the partition coefficient and the longer the analyte is retained on the liquid chromatography column. The components may be detected spectroscopically after elution from the liquid chromatography column by coupling the exit of the column to a post-column detector.
Spectroscopic detectors rely on a change in refractive index, ultraviolet and/or visible light absorption, or fluorescence after excitation with a suitable wavelength to detect the separated components. Alternatively, the separated components may be passed from the liquid chromatography column into other types of analytical instruments for analysis. The analysis outcome depends upon the sequenced arrival of the components separated by the liquid chromatography column and is therefore time-dependent.
The length of liquid transport from the liquid chromatography column to the analysis instrument such as the detector is preferably minimized in order to minimize diffusion and thereby maximize the separation efficiency and analysis sensitivity. The transport length is referred to as the dead volume or extra-column volume.
Capillary electrophoresis is a technique that utilizes the electrophoretic nature of molecules and/or the electroosmotic flow of fluids in small capillary tubes to separate components of a fluid. Typically a fused silica capillary of 100 .mu.m inner diameter or less is filled with a buffer solution containing an electrolyte. Each end of the capillary is placed in a separate fluidic reservoir containing a buffer electrolyte. A potential voltage is placed in one of the buffer reservoirs and a second potential voltage is placed in the other buffer reservoir. Positively and negatively charged species will migrate in opposite directions through the capillary under the influence of the electric field established by the two potential voltages applied to the buffer reservoirs. Electroosmotic flow is defined as the fluid flow along the walls of a capillary due to the migration of charged species from the buffer solution. Some molecules exist as charged species when in solution and will migrate through the capillary based on the charge-to-mass ratio of the molecular species. This migration is defined as electrophoretic mobility. The electroosmotic flow and the electrophoretic mobility of each component of a fluid determine the overall migration for each fluidic component. The fluid flow profile resulting from electroosmotic flow is flat due to the reduction in frictional drag along the walls of the separation channel. This results in improved separation efficiency over liquid chromatography where the flow profile is parabolic resulting from pressure driven flow.
Capillary electrochromatography is a hybrid technique which utilizes the electrically driven flow characteristics of electrophoretic separation methods within capillary columns packed with a solid stationary phase typical of liquid chromatography. It couples the separation power of reversed-phase liquid chromatography with the high efficiencies of capillary electrophoresis. Higher efficiencies are obtainable for capillary electrochromatography separations over liquid chromatography because the flow profile resulting from electroosmotic flow is flat due to the reduction in frictional drag along the walls of the separation channel when compared to the parabolic flow profile resulting from pressure driven flows. Furthermore, smaller particle sizes can be used in capillary electrochromatography than in liquid chromatography because no back pressure is generated by electroosmotic flow. In contrast to electrophoresis, capillary electrochromatography is capable of separating neutral molecules due to analyte partitioning between the stationary and mobile phases of the column particles using a liquid chromatography separation mechanism.
The separated product of such separation devices may be introduced as the liquid sample to a device that is used to produce electrospray ionization. The electrospray device may be interfaced to an atmospheric pressure ionization mass spectrometer (API-MS) for analysis of the electrosprayed fluid.
A schematic of an electrospray system 50 is shown in FIG. 1. An electrospray is produced when a sufficient electrical potential difference V.sub.spray is applied between a conductive or partly conductive fluid exiting a capillary orifice and an electrode so as to generate a concentration of electric field lines emanating from the tip or end of a capillary 52 of an electrospray device. When a positive voltage V.sub.spray is applied to the tip of the capillary relative to an extracting electrode 54, such as one provided at the ion-sampling orifice to the mass spectrometer, the electric field causes positively-charged ions in the fluid to migrate to the surface of the fluid at the tip of the capillary. When a negative voltage V.sub.spray is applied to the tip of the capillary relative to an extracting electrode 54, such as one provided at the ion-sampling orifice to the mass spectrometer, the electric field causes negatively-charged ions in the fluid to migrate to the surface of the fluid at the tip of the capillary.
When the repulsion force of the solvated ions exceeds the surface tension of the fluid sample being electrosprayed, a volume of the fluid sample is pulled into the shape of a cone, known as a Taylor cone 56 which extends from the tip of the capillary. Small charged droplets 58 are formed from the tip of the Taylor cone 56 and are drawn toward the extracting electrode 54. This phenomenon has been described, for example, by Dole et al., Chem. Phys. 1968, 49, 2240 and Yamashita and Fenn, J. Phys. Chem. 1984, 88, 4451. The potential voltage required to initiate an electrospray is dependent on the surface tension of the solution as described by, for example, Smith, IEEE Trans. Ind Appl. 1986, IA-22, 527-535. Typically, the electric field is on the order of approximately 10.sup.6 V/m. The physical size of the capillary determines the density of electric field lines necessary to induce electrospray.
One advantage of electrospray ionization is that the response for an analyte measured by the mass spectrometer detector is dependent on the concentration of the analyte in the fluid and independent of the fluid flow rate. The response of an analyte in solution at a given concentration would be comparable using electrospray ionization combined with mass spectrometry at a flow rate of 100 .mu.L/min compared to a flow rate of 100 nL/min.
The process of electrospray ionization at flow rates on the order of nanoliters per minute has been referred to as "nanoelectrospray". Electrospray into the ion-sampling orifice of an API mass spectrometer produces a quantitative response from the mass spectrometer detector due to the analyte molecules present in the liquid flowing from the capillary.
Thus, it is desirable to provide an electrospray ionization device for integration upstream with microchip-based separation devices and for integration downstream with API-MS instruments.
Attempts have been made to manufacture an electrospray device which produces nanoelectrospray. For example, Wilm and Mann, Anal. Chem. 1996, 68, 1-8 describes the process of electrospray from fused silica capillaries drawn to an inner diameter of 2-4 .mu.m at flow rates of 20 nL/min. Specifically, a nanoelectrospray at 20 nL/min was achieved from a 2 .mu.m inner diameter and 5 .mu.m outer diameter pulled fused-silica capillary with 600-700 V at a distance of 1-2 mm from the ion-sampling orifice of an API mass spectrometer.
Ramsey et al., Anal. Chem. 1997, 69, 1174-1178 describes nanoelectrospray at 90 nL/min from the edge of a planar glass microchip with a closed separation channel 10 .mu.m deep, 60 .mu.m wide and 33 mm in length using electroosmotic flow and applying 4.8 kV to the fluid exiting the closed separation channel on the edge of the microchip for electrospray formation, with the edge of the chip at a distance of 3-5 mm from the ion-sampling orifice of an API mass spectrometer. Approximately 12 nL of the sample fluid collects at the edge of the chip before the formation of a Taylor cone and stable nanoelectrospray from the edge of the microchip. However, collection of approximately 12 nL of the sample fluid will result in remixing of the fluid, thereby undoing the separation done in the separation channel. Remixing causes band broadening at the edge of the microchip, fundamentally limiting its applicability for nanoelectrospray-mass spectrometry for analyte detection. Thus, nanoelectrospray from the edge of this microchip device after capillary electrophoresis or capillary electrochromatography separation is rendered impractical. Furthermore, because this device provides a flat surface, and thus a relatively small amount of physical asperity, for the formation of the electrospray, the device requires an impractically high voltage to initiate electrospray, due to poor field line concentration.
Xue, Q.; Foret, F.; Dunayevskiy, Y. M.; Zavracky, P. M.; McGruer, N. E.; Karger, B. L. Anal. Chem. 1997, 69, 426-430 describes a stable nanoelectrospray from the edge of a planar glass microchip with a closed channel 25 .mu.m deep, 60 .mu.m wide and 35-50 mm in length and applying 4.2 kV to the fluid exiting the closed separation channel on the edge of the microchip for electrospray formation, with the edge of the chip at a distance of 3-8 mm from the ion-sampling orifice of an API mass spectrometer. A syringe pump is utilized to deliver the sample fluid to the glass microchip electrosprayer at a flow rate between 100-200 nL/min. The edge of the glass microchip is treated with a hydrophobic coating to alleviate some of the difficulties associated with nanoelectrospray from a flat surface and which slightly improves the stability of the nanoelectrospray. Electrospraying in this manner from a flat surface again results in poor field line concentration and yields an inefficient electrospray.
Desai et al. 1997 International Conference on Solid-State Sensors and Actuators, Chicago, Jun. 16-19, 1997, 927-930 describes a multi-step process to generate a nozzle on the edge of a silicon microchip 1-3 .mu.m in diameter or width and 40 .mu.m in length and applying 4 kV to the entire microchip at a distance of 0.25-0.4 mm from the ion-sampling orifice of an API mass spectrometer. This nanoelectrospray nozzle reduces the dead volume of the sample fluid. However, the extension of the nozzle from the edge of the microchip exposes the nozzle to accidental breakage. Because a relatively high spray voltage was utilized and the nozzle was positioned in very close proximity to the mass spectrometer sampling orifice, a poor field line concentration and a low efficient electrospray were achieved.
In all of the above-described devices, edge-spraying from a monolithic chip is a poorly controlled process due to the inability to rigorously and repeatably determine the physical form of the chip's edge. In another embodiment of edge-spraying, ejection nozzles, such as small segments of drawn capillaries, are separately and individually attached to the chip's edge. This process is inherently cost-inefficient and unreliable, imposes space constraints in chip design, and is therefore unsuitable for manufacturing.
Thus, it is also desirable to provide an electrospray ionization device with controllable spraying and a method for producing such a device which is easily reproducible and manufacturable in high volumes.